In a typical transit system operative with commafree, binary coded vehicle control signals, speed command information is transmitted on multiple message frequencies (e.g. F.sub.1 and F.sub.2) which typically represent binary logic conditions (e.g. 1 and 0). A digital frequency modulation method (FM), frequency shift key modulation method (FSK), or phase shift key modulation method (PSK) is employed to carry the binary coded, speed command information to the transit vehicles. Before the information transmitted by the message frequencies can be utilized, it must be decoded according to the particular modulation method used by the system. This decoding must be performed in a manner such that extraneous signals will not cause the vehicle to operate in an unsafe manner.
In attempting to achieve this safety requirement, vehicle command signal decoders of the prior art used a limiting amplifier to establish a "capture effect", by which a coded message signal of large amplitude could screen out extraneous signals of lower amplitude. The gain of the limiting amplifier was high enough so that any input signal of a predetermined minimum value would result in a limited maximum output of the same polarity. Therefore, as long as the amplitude of the message signal was large enough to maintain the minimum value of the input signal while, at the same time, offsetting any other input signals, the polarity of the limited output of the limiting amplifier was determined by the polarity of the message signal. For example, an input message signal whose amplitude was at least twice as large as the amplitude of any other input signal, i.e., noise or transient filter responses, would screen out the other signals so that the polarity of the limited maximum output of the limiting amplifier would be determined by the polarity of the message input signal.
A problem with the prior art decoders was caused by the high gain of the limiting amplifier. For example, if the output of a filter in the prior art decoder were to contain a transient response while the message frequency being transmitted was outside the bandwidth of the filter, the limiting amplifier could recognize this transient response as a valid input and amplify it to the limited level so that the decoder would decode inaccurate information.
The high gain of the limiting amplifier could also cause errors in detection where the large amplitude message signal was lost. When the large amplitude message signal was not present to screen out the smaller, extraneous signals, the limiting amplifier could amplify these extraneous signals also causing the decoder to decode inaccurate information.
Also, the failure of a filter could allow a signal outside the bandwidth of the filter to reach the input of the limiting amplifier where it could be amplified and again cause the decoder to decode inaccurate information.
It was the purpose of the present invention to provide a transit vehicle signal decoder that could improve transit system safety by eliminating the decoding errors caused by the use of limiting amplifiers.